Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

677
FXUS63 KLOT 031428
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
928 AM CDT Fri Jul 3 2026

.KEY MESSAGES...

- Hot and humid conditions will continue today, though the
  highest heat indices around 100 degrees will likely focus
  near/south of the Kankakee River. On Independence Day, peak
  heat indices of ~95-100F are expected near and south of I-80.

- Waves of thunderstorms are expected through the weekend,
  though there will be many dry hours. Any storms today will be
  capable of producing damaging winds. There will also be a
  threat for localized flash flooding with any focused corridors
  of storms through the weekend.

&&

.UPDATE...
Issued at 928 AM CDT Fri Jul 3 2026

Per a recent hand surface analysis, a frontal boundary extends
from southern Iowa northeastward into northern Illinois and
across far southern Lake Michigan. The frontal boundary
represents the northern terminus of moisture-laden instability
that was responsible for the overnight thunderstorms that
produced locally damaging winds and swaths of torrential
downpours across the Chicago metropolitan area. Mostly sunny
skies in the area are helping temperatures rise quickly this
morning, with several locations already moving through the lower
80s as of the issuance of this AFD update. When combined with
dew points in the low to mid 70s (especially south of the
aforementioned frontal boundary), it`ll feel uncomfortably hot
and humid as the morning progresses.

The main focus today is on two windows for thunderstorms. While
a complex of storms in central Iowa has been on a steady
weakening trend (a consequence of a lack of a cold pool to
activate the downstream instability axis), have noted new
thunderstorms developing just west of the Mississippi River
within the past 30 minutes. These thunderstorms will be better
positioned to take advantage of the hot and humid airmass across
northern Illinois. Accordingly, suspect they will continue to
intensify and increase in coverage while moving into northern
Illinois later this morning and into the early afternoon. A
pocket of locally enhanced 45-50 kt of flow at 4-5 km (per the
DVN RAOB) will support sufficient east-northeasterly convective-
layer to support clustering of thunderstorms with a threat for
locally damaging winds. At this point, the threat area looks to
favor areas along and north of I-80 generally between 11 AM and
3 PM. This activity will likely reinforce the position of the
frontal boundary somewhere near or north of Interstate 88.

The second window for storms will be later this evening and
overnight as a trailing upper-level wave (currently moving into
western Iowa) interacts with the stalled frontal boundary and
replenishing instability. Do think coverage of storms may be
higher during the second window along a west-to-east axis
initially north of I-80 before expanding across much of the area
through the overnight hours. The time window for the second
round of storms looks to be roughly from 6 to 10 PM north of
I-80, and then areawide from 10 PM onward. Like during the first
time window, the strongest storms during the second time window
may be capable of locally damaging winds and torrential
downpours. Needless to say, the second round of storms could be
quite a headache for all the outdoor festivities planned across
the area.

One final note - Did opt to downgrade the Extreme Heat Warning
south of I-80 to a Heat Advisory. With peak heat indices this
afternoon expected to top off right around or just above 100
degrees, feel we are just not meeting the very high bar that is
required with an Extreme Heat Warning. However, in deference to
the long duration of the heat (day 5 for that area), think a
heat Advisory is still a good call.

Updated products are coming soon.

Borchardt

&&

.DISCUSSION...
Issued at 333 AM CDT Fri Jul 3 2026

Earlier storms have exited the area to the east with the
associated outflow boundary having managed to push as far south
as Paxton, IL bringing temperatures into the lower 70s
areawide. While a stray shower or storm can`t be ruled out
overnight, most areas are expected to remain dry.

The residual outflow boundary from storms early this morning
is anticipated to lift back to the north some though exactly
where remains uncertain. Opted to maintain the Extreme Heat
Warning for counties along and south of the Kankakee River
Valley, where heat indices may reach 105 degrees this
afternoon. Farther north will still be hot and humid, but not to
the extent of the past several days with forecast peak heat
indices in the 95-100 degree range.

The shower and thunderstorm potential trends for today are also
uncertain, as is often the case in these types of patterns.
First, we are monitoring storms over northeast Nebraska into
central Iowa ongoing early this morning. This feature is
expected to progress east with time, likely in a weakening
state. However, an MCV appears to be developing across northwest
Iowa which, paired with a likely convectively augmented mid-
level vort, may serve as a forcing mechanism for shower and
thunderstorm development into northern Illinois in the
afternoon. Additionally, the aforementioned outflow boundary may
serve as a forcing mechanism for shower and thunderstorm
development in the afternoon. Again, questions remain as to
exactly how far north the boundary will lift this afternoon. It
is tough to say whether the late evening timeframe will be
shower and storm free given the potential for additional
redevelopment along any potential residual outflow boundaries
from any storms that develop earlier in the day. While shear is
not especially strong, ample instability in place will be
supportive of damaging wind gusts with any storms today/tonight.
Any training corridors of storms could also lead to flooding.

Going forward into Independence Day, a larger disturbance is
forecast to move across the region bringing another round of
showers and storms, currently favoring the afternoon to early
evening timeframe. The northern periphery of the better
instability will likely have shifted south some in response to
the Friday/Friday night rounds of storms. Still can`t rule out
locally strong to damaging wind gusts with any storms that do
develop, with perhaps a locally higher threat south of I-80.
While it is important to not latch onto any one model run, it is
worth noting that the latest HRRR (6Z) attempts to develop more
robust storms north of I-80 on Saturday as the wave moves
through (likely owing to farther north position of earlier
rounds of convection). Accordingly, the message will remain that
additional rounds of storms can be expected (some potentially
strong to locally severe).

Sunday will feature yet another mid-level wave moving across the
region due to the flattening of the upper ridge. This will bring
additional rounds of showers and isolated embedded (non-severe)
storms with greatest coverage favored in the afternoon-early
evening. Temperatures will be closer to normal for early July
with highs in the lower to mid 80s.

Beyond the weekend the upper ridge begins to build back north
slightly with the embedded waves moving to our north. Meanwhile
another wave (or waves) will move within the ridge to our south
(and potentially stall out). Overall this favors shower/storm
coverage focusing mainly north or south of the area into mid-
week. Shower and storm chances return later in the week as the
ridge begins to break down again.

Petr

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 624 AM CDT Fri Jul 3 2026

Forecast concerns for the terminals...

- Scattered thunderstorms likely this afternoon and evening. A
  few of which could be strong with gusty winds in excess of 30
  kts and heavy downpours.

- Another period of showers and storms is expected overnight
  into Saturday morning.


The earlier line of showers and storms has moved away from the
terminals giving us a mostly sunny start to our Friday morning.
While winds have been varying between 170-190 early this
morning, winds will settle into a west-southwest direction this
afternoon with speeds around 10-12 kts.

The main forecast concern for today will be timing out the
arrival of the thunderstorm complex ongoing in central IA.
Current trends suggest the complex arriving in the 19-21z
timeframe (earliest at RFD and latest at GYY), but with the
outflow boundary from last night`s storms still lingering in
northeast IL and northwest IN cannot rule out some isolated
activity tries to get going a bit sooner. Regardless, once
storms arrive they should persist for at least 2-3 hours before
exiting the terminals. Given the humid and unstable air mass
forecast to be in place, any stronger storms will be capable of
gusty winds in excess of 30 kts and heavy downpours that will
result in reduced visibilities and some lower ceilings.

A break is expected to develop behind the storms this
afternoon/early evening. Though a lot of uncertainty resides on
how long the break will last as another thunderstorm complex
should be ongoing in IA and working its way across IL and IN
overnight. Due to the uncertainty in arrival/development of
storms tonight have opted to introduce a lengthy PROB30 in the
TAFs for this potential but suspect the actual duration of
showers and storms overnight to be in the 3-4 hour range. With
the loss of diurnal heating storms tonight should mainly be a
heavy rain threat, but some locally gusty winds cannot be ruled
out.

Outside of the showers/storms today and tonight, expect VFR
conditions to prevail with otherwise southwest winds around 5-10
kts. However, winds may become variable around and behind
thunderstorms for a couple of hours before settling back into a
southwest direction.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory until 7 PM CDT this evening for ILZ023-ILZ032-
     ILZ033-ILZ039.

IN...Heat Advisory until 7 PM CDT /8 PM EDT/ this evening for
     INZ010-INZ011-INZ019.

LM...None.

&&

$$

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